For improving fuel economy, improving a heat efficiency is raised as one method. Note that, for improving a net heat efficiency, increasing compression ratio of mixture, leaning of mixture and the like are raised for example.
In the case of the leaning of mixture; the more leaned the mixture is, the less stabilized the combustion is. For stabilizing the combustion, the following operations are taken: Increasing flame propagation speed by causing a strong gas flow such as swirl, tumble and the like to an area in the combustion chamber to thereby shorten the combustion period.
Moreover, in the case of an ignition plug as well; for securing a stabilized ignition, an electrode discharge part is rendered to have a smaller diameter and an iridium for improving wear-proof is used.
Moreover, a multiple-point ignition is used for shortening a flame propagation distance, to thereby stabilize the combustion.
Moreover, Japanese Patent Application Laid-Open No. Showa 58 (1983)-195071 and Japanese Patent Application Laid-Open No. Showa 63 (1988)-173852 each disclose a multiple-point ignition, and report a technology using a laser, instead of a conventional ignition plug, which laser is capable of making a stable ignition even in a state of a strong gas flow.
Moreover, the laser igniting method can arbitrarily set ignition positions. Therefore, in this respect, the laser igniting method is capable of making the multiple-point ignition. Moreover, the laser has features not found in any other ignition system(s), i.e., spending a very short time for ending ignition, specifically, the time required for the ignition is tens of nano seconds (about one thousand nano seconds according to the conventional ignition plug).
As such, the laser igniting method having the above features can improve combustion of mixture to a great extent.
For example, in a combustion chamber of an automobile engine, increasing the compression ratio of mixture, as the case may be, involves a knocking. In this case, however, varying ignition positions or implementing a multiple-point ignition can narrow down an end gas region, thus suppressing the knocking.
Moreover, the swirl, tumble or the like is caused in an engine for implementing a lean burn having a high specific heat ratio, so as to increase the flame propagation speed. When the above operation is too strong, however, the ignition is sometimes unlikely.
In this respect as well, the laser igniting method which ends the ignition after tens of nano seconds can make a stable ignition.
Concerning the energy required for ignition, the Japanese Patent Application Laid-Open No. Showa 58 (1983)-195071 and the Japanese Patent Application Laid-Open No. Showa 63 (1988)-173852 each propose a technology for improving ignition efficiency by focusing a light in the mixture.